A logic level is one of several states that a digital signal can possess, expressed as a DC (direct-current) voltage with respect to electrical ground. Usually, the term refers to binary logic in which two levels, or states, can exist: logic 1 (also called the high state) and logic 0 (also called the low state).
In most circuits, logic 1 is represented by approximately +5 V (positive 5 volts) relative to ground, while logic 0 is represented by approximately the same voltage as ground (0 V). This system is called positive or active-high logic. In some circuits the two voltage levels are reversed, so that the higher voltage represents logic 0 and the lower voltage represents logic 1. This system is known as negative or active-low logic.
In most practical systems, there is some room for error in the logic voltages. For example, in an active-high circuit, logic 1 might be represented by any voltage between +3.5 V and +6.5 V, while logic 0 might be represented by any voltage between -1.0 V and +2.0 V. A signal between +2.0 V and +3.5 V would not be recognized as either low or high, and would be rejected as invalid.
In practical binary circuits, logic levels are handled and manipulated by electronic switches, called logic gates, connected in massive arrays to perform digital calculations and operations. Binary logic serves as the basis on which nearly all digital devices, including computers, operate.
Some circuits work with more than two digital levels. A system based on trinary logic has three levels, representing digits or states called -1 (false), 0 (neutral), and +1 (true); most others have some higher power of two levels, such as four, eight, 16, 32, 64, and so on.